1. Field of the Invention
This invention relates to a method of and a device for crystal diameter measurement to be used in an apparatus for automatically controlling single crystal growth by the CZ technique.
2. Description of the Related Art
A conventional example of such a crystal diameter measuring device will be described with reference to FIGS. 3 to 5, which will also be referred to in an illustration of an embodiment of the present invention. In a conventional device, a luminous ring 23, which is formed in the border between the surface 22S of a melt 22 and a silicon single-crystal 32 which is pulled up, is photographed by a CCD camera, and the resulting video signal is binary-coded to obtain a binary image 68. The inner diameter D.sub.I of the luminous ring image 70 contained in this binary image 68 is detected and multiplied by a constant, thereby obtaining the actual lower end diameter D.sub.R of the silicon single-crystal 32. When the length of the line 72 through the center of the luminous ring image 70 is of actual size, the difference between the inner diameter D.sub.I and the real diameter D.sub.R is almost negligible. Further, this difference has substantially nothing to do with the real diameter D.sub.R, the surface inclination, etc., of the silicon single-crystal 32. Accordingly, with this conventional diameter measuring device for automatically controlled crystal growth, the real diameter D.sub.R can be measured with high accuracy.
In a single crystal growth apparatus using the CZ technique, an automatic control method has been established for use in those stages from beginning with the growth of the corn region of the crystal and thereafter. However, the stages for forming the necked region of the crystal, beginning with the immersion of the seed crystal in the melt to the stage for the growth of the corn region, are manually conducted by a skilled operator to terminate any dislocation in the necked region of the growing crystal. To effect this, a high level of manual skillfulness is required. For example, the diameter of the necked region of the growing crystal must be limited to a range of 2 to 5 mm, and the crystal has to be pulled at a relatively high rate of not less than 2 mm/min. Furthermore, the absolute value of the deviation in diameter control has to be around 0.5 mm or less, and the necked region has to be allowed to grow in a length ten times larger than the diameter thereof. The operation of forming the necked region and the subsequent operation of augmenting the crystal diameter, while imparting a desired shape to the crystal without leaving any dislocation therein, are so difficult, that even a skilled operator can perform it with a failure rate of as much as 10 percent of all the trial. If the diameter is excessively narrowed, a disconnection occurs in the region between the surface of the melt and the lower end of the growing crystal, thereby making it impossible to continue the growth of the necked region. If such a disconnection is avoided somehow, the cylindrical section grown afterwards cannot be supported because of the weakness of the necked region with excessive narrowness in diameter. An overly large crystal diameter is not desirable in the necked region, either, since that would not sufficiently allow for the termination of any dislocation, thereby making it impossible to move on to the next stage for the growth of the corn region.